The present invention relates to brush seal arrangements for sealing between pressure regions having a pressure drop in excess of the capacity of individual brush seals and particularly relates to a brush seal arrangement for steam turbine high pressure end seals wherein the pressure drop across any one brush seal is within the sealing capacity of the one brush seal.
In turbomachinery, brush seals have been previously proposed and constructed for use between relatively moving parts having a pressure differential across the parts. In steam turbines typically comprising a stationary housing and a rotor mounting a plurality of buckets wherein steam under pressure passes through a steam path to rotate the buckets and rotor, there are a number of locations requiring seals between relatively moving parts. For example, low pressure end seals in a steam turbine frequently employ a combination of brush and labyrinth seals mounted on a plurality of arcuate segments for sealing between the high and low pressure regions on opposite axial sides of the sealing segments. Brush seals, of course, typically include a plurality of bristles, for example, metal or ceramic bristles usually disposed between a pair of backing plates and arranged in an arcuate segment such that the tips of the bristles engage the surface, e.g., a rotary surface, to be sealed. While advantageous, brush seals per se, however, have inherent limitations. These limitations may be exemplified by reference to the high pressure end seals in a steam turbine. Inlet pressures for large steam turbines are typically in a range of 1800-3500 psig. At the high pressure end seal location, steam leakage flow is driven by the pressure drop from the control stage outlet pressure to the next fixed pressure, typically the high pressure section exhaust pressure of approximately one-quarter inlet pressure. The pressure drop may exceed 2000 psig. Steam flow through this leakage path is reduced by a series of labyrinth seal rings disposed between the two pressure regions. Existing single-stage brush seals, however, are capable of sealing across pressure drops of up to 400 psig, which can reasonably be extended to approximately 600 psig by reducing the length of the unsupported bristles at the seal inner diameter, i.e., the fence height. Fence height reduction, however, has its own limitation because adequate clearance must be given for expected seal/rotor transients.
Use of brush seals in series to share the total overall pressure load, i.e., the total differential pressure drop between the high pressure control stage outlet and the HP section exhaust pressure is impractical because there is a natural tendency for the pressure distribution to be biased toward the downstream seals. Thus, the downstream seals, if formed of brush seals, would take a disproportionate or larger percentage of the total pressure load and may exceed the brush seal capacity. Consequently, only labyrinth-type seals are typically used at the high pressure end seal location in a steam turbine, i.e., at those locations where the pressure drop is in excess of the capacity or capability of a conventional brush seal. Accordingly, there is a need for an arrangement of brush seals for sealing across pressure drops in excess of the capability of individual brush seals.
In accordance with a preferred embodiment of the present invention, there is provided a brush seal arrangement for sealing between high pressure regions having a total pressure drop in excess of any individual brush seal sealing capability. To accomplish the foregoing, and with reference to an exemplary embodiment of the sealing arrangement in a steam turbine high pressure end seal, the overall pressure drop which is in excess of the sealing capacity of any individual brush seal is divided into several discrete pressure drops of lesser magnitude. Each pressure drop lies within the design capacity of the brush seal. To accomplish this, the division of the overall pressure drop into several discrete lesser pressure drops includes establishing pressure regions on opposite sides of individual brush seals. These pressure regions are at relatively substantially fixed predetermined pressures and are in communication with other locations of the turbine system that are at such intermediate pressures. For example, at the high pressure end seal of a steam turbine, an intermediate pressure region may comprise an intermediate pressure connection at an interstage location or a connection to an extraction to a heater above the reheat point (HARP). In this manner, the pressure drop across any one brush seal between adjacent pressure regions may be set within the design capacity of the brush seal. The result is that the entire high pressure end seal region or a portion of that region can be effectively sealed using brush seals which typically offer more significant leakage reduction as compared to conventional labyrinth seals usually used to seal these locations in steam turbines.
In a preferred embodiment according to the present invention, there is provided a turbine comprising a stationary component and a rotary component, first and second seal assemblies axially spaced from one another between the stationary component and the rotary component, each of the first and second seal assemblies including a brush seal for sealing across a pressure differential, each the brush seal having a predetermined maximum differential sealing pressure defining a sealing capacity therefor, the turbine including a first high pressure region upstream of the first seal and a second lower pressure region downstream of the second seal, the first and second pressure regions having a total pressure drop in excess of the sealing capability of each brush seal of the first and second brush seal assemblies, at least one pressure region intermediate the first and second pressure regions and between the first and second seal assemblies, the one intermediate pressure region having a substantially fixed predetermined pressure such that the predetermined sealing capacity of each brush seal between one of (i) the first and intermediate pressure regions and (ii) the intermediate and the second pressure regions is not exceeded.
In a further preferred embodiment according to the present invention, there is provided a steam turbine comprising a stationary component, a rotary component within the stationary component and having a plurality of buckets, a steam inlet for supplying steam to the buckets to rotate the rotary component, the turbine having a high pressure end adjacent the steam inlet, first and second seals axially spaced from one another between the stationary component and the rotary component at the high pressure end, each of the first and second seals including a brush seal, the turbine including a first steam pressure region exposed to high pressure steam on a side thereof upstream of the first seal, a second steam pressure region downstream of the second seal, the first and second pressure regions having a total pressure drop in excess of the sealing capability of each of the first and second brush seals, at least one pressure region intermediate the first and second pressure regions and between the first and second seals having a substantially fixed predetermined pressure such that a pressure differential across one of the first and second seals is within the sealing capability of the one seal.